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Determining the Mole Ratios in a Chemical Reaction

Figure from experiment 9 from Advanced Chemistry with Vernier


A balanced chemical reaction equation gives the mole ratios of the reactants and the products as coefficients. When some of the chemical formulas are not known, an experiment must be conducted to help determine the mole ratios.

This experiment uses two common substances as the reactants: hypochlorite ion (OCl) from household bleach and thiosulfate ion (S2O32–), the active ingredient in a photographic “fixer” solution used to develop film. In the reaction, hypochlorite ions oxidize the thiosulfate ions according to the unbalanced and incomplete reaction equation below.

A{\text{ OC}}{{\text{l}}^-} + B{\text{ }}{{\text{S}}_{\text{2}}}{{\text{O}}_{\text{3}}}^{{\text{2}}-} \to {\text{products}}

It is possible to identify the coefficients, A and B, for the reactants, without knowing the products of the reaction. The process that you will use to determine the coefficients is called continuous variations. You will prepare a series of mixtures of the two reactants. Each mixture will have the same total volume and the same total number of moles of reactants. The reaction is exothermic, thus the mixture that generates the most heat energy will be the reaction that completely consumes both the hypochlorite and the thiosulfate ions. You will use this mixture to establish the coefficients, and therefore the mole ratio, for the reaction.


In this experiment, you will

  • Measure the enthalpy change of a series of reactions.
  • Determine the stoichiometry of an oxidation-reduction reaction in which the reactants are known but the products are unknown.

Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

Option 1

Option 2

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

Standards Correlations

See all standards correlations for Advanced Chemistry with Vernier »

Experiment 9 from Advanced Chemistry with Vernier Lab Book

<i>Advanced Chemistry with Vernier</i> book cover

Included in the Lab Book

Vernier lab books include word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.

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